Welsh painter Kyffin Williams is often regarded as the defining artist of Wales during the 20 century, with his works known to fetch tens of thousands of pounds.

Detective Constable Ray Swan, from Scotland Yard's Art and Antiques Unit, said: "Whoever stole this painting is likely to try to sell it. I urge anyone who is offered the painting to report the matter to us immediately, or contact us if you know anything about the theft or the painting's whereabouts."Anyone with information can call the Art and Antiques Unit on 020 7230 2150.